The Essential Guide to Image Processing- P7:We are in the middle of an exciting period of time in the field of image processing. Indeed, scarcely a week passes where we do not hear an announcement of some new technological breakthrough in the areas of digital computation and telecommunication. | 180 CHAPTER 8 Color and Multispectral Image Representation and Display TABLE Qualitative description of luminance levels. Description Lux Cd m2 Footcandles Moonless night 1 0 6 10 Full moon night 1 0 3 10 Restaurant 100 9 Office 350 33 Overcast day 5 000 465 Sunny day 200 000 18 600 Spatial sampling is done using a regular grid. The grid is most often rectilinear but hexagonal sampling has been thoroughly investigated 6 . Hexagonal sampling is used for efficiency when the images have a natural circular region of support or circular symmetry. All the mathematical operations such as Fourier transforms and convoutions exist for hexagonal grids. It is noted that the reasons for uniform sampling of the temporal dimension follow the same arguments. The distribution of energy in the wavelength dimension is not as straightforward to characterize. In addition we are often not interested in reconstructing the radiant spectral distribution as we are for the spatial distribution. We are interested in constructing an image which appears to the human observer to be the same colors as the original image. In this sense we are actually using color aliasing to our advantage. Because of this aspect of color imaging we need to characterize the color vision system of the eye in order to determine proper sampling of the wavelength dimension. COLORIMETRY To understand the fundamental difference in the wavelength domain it is necessary to describe some of the fundamentals of color vision and color measurement. What is presented here is only a brief description that will allow us to proceed with the description of the sampling and mathematical representation of color images. A more complete description of the human color visual system can be found in 7 8 . The retina contains two types of light sensors rods and cones. The rods are used for monochrome vision at low light levels the cones are used for color vision at higher light levels. There are three types of cones. Each type is .
